Running a one-shot is a nice introduction to running D&D, as they are in many ways less pressure than running a campaign session. Not only is there much less in the way of lore and different story arcs that you have to remember and weave in, but by and large they are also more predictable than a typical campaign session. As such, a one-shot is a perfect intro to DMing and gives you a chance to see if it is for you, without you having to commit to a multiple session campaign.
*N.B. The Amazon links (marked with an *) on this page are affiliate links. This doesn't cost you any extra, but I make a small commission from any sales - which helps support the site :)
If you want to keep prep to a minimum, there are some really good books which have collections of oneshots in. Personally, I own Candlekeep Mysteries(*) and both Prepared(*) and Prepared 2(*) for days when I want to run a session but don't have a lot of prep time or creative energy. For the most part, though, I prefer to create and run my own oneshots. If you're in the same boat, here's some tips to make that as straightforward as possible.
Make a dungeon
The first one-shots I ran tended to be 4 or 5 room dungeons. These are straightforward to run as the players choices are confined within a single space - making it both easier for you to prep what they'll come across, and also to manage the pacing of the session, as you always know how much ground your players have covered and what is still to come. To try and make things enjoyable for a range of different players, I will also try to have a mix between combat, puzzles and social encounters within those rooms - perhaps also prepping maps, minis and props depending on how much time I have available.
Start with an idea
A good one-shot often comes out of just a single idea. Like any sane person, I am a massive fan of the 1999 cinematic masterpiece The Mummy. Fortunately, my partner is too, so for her birthday last year I prepped one-shot inspired by the film. I decided that a 4-room dungeon would work pretty well for a 2-hour session (with a party of 4-6 people) so I went with that structure, with each room containing a canopic jar that the party needed to find before Imhoteph did.
In the first room the party had to fight a bunch of mummies who came out of the frescos on the walls (they could have tried to talk their way past them, I suppose - but that thought never occurred to my players...). In the second room I had a logic puzzle. The third room involved a social encounter between the players where they had to each tell a secret to get through. Finally, the last room involved a big battle in a subterranean chamber against Imhoteph, as he was trying to bring Anuk-su-namun back to life. I guess you could say that a final escape sequence after that battle, to get out of the chambers before the place was consumed by the sands, would actually count as a fifth room.
Knowing the overall shape of the session and the time constraints I had, meant I could gently hurry players along a little if they were falling behind schedule (perhaps by having a couple of the mummies fleeing during the first combat, or giving them a few hints on the logic puzzle etc).
A four or five room dungeon such as this then makes a fun and varied one-shot, where you as DM have basically complete control of what your players will run into, and of the session pacing. As such, they're perfect for a beginner DM.
Write a good pre-amble
In a regular campaign, you have several sessions to establish the tone of your game, the world-lore, and the overall ambitions and story arcs of your players. In a one-shot, you have only a couple of minutes to set up all of this. As such, you might need a little more exposition than usual.
In general, I hate long, rambling DM monologues and try to keep my descriptions of places or situations to only about 30 seconds (while having additional information available to players who ask/ search for it etc). With a one-shot, I will happily stretch this to a couple of minutes with a good opening pre-amble. You have this one opportunity to get your players engaged in the world, to explain the gravity of their character's role within it, and in general just to get them excited about the session - so make sure you use this time well!
I ran a single-player one-shot for my partner a while back that introduced a new world, based in a remote, run-down desert city that had become a bit of a backwater since its mining economy collapsed. It was pretty much inspired by Dune. I used the preamble to give a short synopsis of the world/ city they were in, the relationship between her character and a couple of important NPCs, and a couple of establishing scenes that led her character from the city university, to her mother's bedside in the hospital, and eventually towards the abandoned mine, where the one-shot began.
This degree of railroading I would never normally consider within a campaign, and the monologue probably lasted for 2-3 minutes. In managing to establish the character's place within the world though, as well her reason for caring about what was going on, this couple of minutes really was time well spent.
Railroad while seeming not to
In general, I rail-road a lot more in my one-shots than in normal campaign sessions. Whilst in a campaign it's fun for your players to kick back and start chatting to random locals in the tavern and play some gambling games instead of going on a quest, in a one-shot you don't have the luxury of time, so it's got to get to the point a bit quicker! As such, I will give my players more nudges in the right direction. Often though, nudges aren't necessary, and you can redesign the geography of your encounter to suit the story you want to tell.
In the one-shot in the mine I just mentioned, after descending through the mineshaft, my partner's character came to a subterranean cavern. She approached a cliff with a 200 foot drop to an underground river running beneath the mine (which was going to be involved in the climax of the session). At the cliff edge, there were two pathways running left and right, leading to tunnels taking her character deeper into the bowels of the cave system which made up the mine.
Given the 200ft fall, I felt certain that her character would take one or other of the clifftop paths, and had a sequence of encounters planned along those pathways, which would take her deeper into the mine and increase the tension as the session went on. Of course, she decided instead to cast feather fall and descend to the river at the bottom of the cliff, something I had not planned for at all.
No worries - at that stage I mentioned that there was a small pathway leading along the river, that led back into the caves at that level. When she followed that, each encounter which had been arranged to take place on the pathways at the top of the cliff merely happened in the caves at the bottom of it instead.
For the player, the experience of this kind of approach, compared to the alternative of me stubbornly trying to railroad my players down one path at the top of the cliff, is poles apart - yet there's no extra prep involved. In short, this approach allows the player the illusion of complete freedom, while still giving me control over the session. Of course, this advice would ring true for a regular campaign session as well - but given that with a one-shot you really are constrained by time and need to get to a certain pre-ordained finish point by the end of your allotted play-time, having that flexibility to rearrange things on the fly is really helpful.
Use pre-generated characters
Creating single-use character sheets can actually be the single most time-consuming aspect of creating a one-shot (I always do this for my players, rather than asking them to put in the time to create someone who will never be used again). As such, I tend to use an online character generator to do the heavy lifting. Once I have the character sheets, I'll then likely tailor the characters' backstories to make them interesting and suitable for the adventure I'm planning to run. This way I still get to do all the interesting stuff - but the time-consuming part of figuring out stat-blocks etc is all done for me.
Expand as you get more confident
As you get more experience running one-shots and feel more comfortable introducing unpredictable elements into the session, you can start to expand outside of the rigid 4 or 5 room dungeon template. In my experience, it works quite well to have a few well-prepared elements to your one-shot, but with some degree of freedom in moving between them. A strong storyline creating urgency and a clear goal will also help you to push your players in the right direction if they meander too far for too long.
A recent one-shot I ran took place across an entire city, with little set pieces planned in a couple of places which my players were told they had to get to - but also gave them a degree of freedom in how they moved between these. It was set during the fall of an ancient city (as a spin off to the main campaign I'm running) - so when moving from a temple to the jail across town, they had the option to move at city level and encounter the invading forces, or try to move through the sewer system (a little anachronistic in ancient Greece, I'll admit).
In the end they climbed an aqueduct and traversed the city that way. I also threw in a couple of hooks to the main campaign along the way, which they could either engage with or not as they saw fit. Leaving these things only loosely planned out not only gave my players more agency, but also made things more interesting for me - yet knowing the main story beats and where it was heading still made it manageable for me to keep the story moving and ensure that we got to the climax of the session in good time.
So, there you go. What's stopping you from coming up with your own ideas and running your very own custom one-shot? It's a brilliant intro to DMing, and may well lead to the urge to run an entire campaign before long!